In the treatment of dental caries and defects, a dental composite material comprising a resin and an inorganic filler (hereinafter referred to as a composite material in some cases) has been widely used in place of conventional metal materials. Such a composite material is excellent in mechanical strength, aesthetic appreciation, durability, and the like, and the composite material is useful in the morphological restoration of a desired site and as an adhesive for the adhesion of a dental material to teeth.
In recent years, those materials having an additional ion-releasing function, especially fluorine ion-releasing function, to these composite materials have been clinically used. By supplying fluorine ions to the teeth after the restoration, these materials have additional functions of changing hydroxyapatite, which is a main constituent of teeth, to a higher acid-resistant fluoroapatite, thereby increasing the resistance to caries of teeth, accelerating the calcification of mineral components in the saliva and tissue fluids or the like. Therefore, opportunities for the clinical use of these materials for preventing the onset and recurrence of dental caries have been increasing.
As those conventionally used as a filler acting as a fluorine ion-releasing source in these composite materials, there have been known fluorine-containing glass such as fluoroaluminosilicate glass, fluorine-containing organic polymer compounds (Japanese Patent Laid-Open No. Hei 5-85915), fine particles obtained by sublimating the solvent from a metal fluoride solution (Japanese Patent Laid-Open No. Hei 2-258602), and the like. Also, more recently, there has been proposed a metal fluoride having a polysiloxane coating layer on the surface thereof (Japanese Patent Laid-Open No. Hei 10-36116). In this publication, it is disclosed that the composite material comprising the metal fluoride is excellent in mechanical strength and amount of fluorine ions released. This technique is basically thought to be encompassed within the scope of the technique in which the surfaces of metal fluoride particles are treated with a silane-coupling agent.
In a case where a composite material comprising sodium fluoride or fluoroaluminosilicate glass as a filler is filled in a defective site of the tooth and then subjected to finish polishing, fluorine ions are effectively released in an oral cavity upon the contact of these orally exposed particles with water. However, the filler itself, at the same time, is gradually dissolved, resulting in detachment of the filler from the surface of the composite material or generation of voids in the inner portion of the filler, whereby the aesthetic appreciation and the mechanical strength of the composite material are lowered. The fluoroaluminosilicate glass, which is the most frequently used material, is likely to dissolve when the degree of acidity of water to which the glass is exposed is increased, so that dissolution is caused within the changing range of pH in the oral cavity (pH 5 to 7).
On the other hand, the fluorine-containing polymer compound does not cause deterioration of the surface properties or lowering in the mechanical strength of the composite materials as mentioned above. However, since the polymer compound itself has low transparency, the aesthetic appreciation of the composite material is lowered. Also, since the releasing rate of the fluorine ions from the composite material is gradual, the clinical effect would be less likely to be exhibited at an early stage.
When the fluorine ion-releasing source is used as a filler for the composite material, it is important that the composite material has appropriate transparency in order to ensure aesthetic appreciation of the composite material. In order to ensure the aesthetic appreciation of the composite material, it is effective to reduce the difference in visible light refractive indices between an X-ray opaque glass such as fluoroaluminosilicate glass, barium-containing glass, strontium-containing glass or lanthanum-containing glass, which is used as the filler, and the resin constituting the composite material. However, since sodium fluoride, which is often used as a fluorine ion-releasing source, has a drastically low photorefractive index than that of the filler or the polymer compound mentioned above, the transparency of the composite material is lowered when added in large amounts, thereby undesirably worsening the aesthetic appreciation of the composite material.
Also, in the technique using a metal fluoride having a polysiloxane coating layer on the surface thereof disclosed in Japanese Patent Laid-Open No. Hei 10-36116 mentioned above, there are some defects in that the metal fluoride powder used has a large particle size, so that large voids are generated due to the dissolution of the metal fluoride particles in the oral cavity, and that the transparency of the composite material comprising the metal fluoride is lowered.
An object of the present invention is to provide a filler which has high water resistance and high acid resistance, and high releasing abilities of ions such as fluorine ions, and can maintain high aesthetic appreciation for a long period of time, and a dental composite material comprising the filler, excellent in water resistance, acid resistance, ion releasability and aesthetic appreciation.